1. Field of the Invention:
The present invention relates to novel polyamide compositions comprising a fire-resisting amount of red phosphorus and intended especially for the manufacture of articles for the electrical and electronics industries. More particularly, this invention relates to polyamide compositions for the production of fire-resistant articles presenting a good compromise of properties, notably with regard to fire resistance, resilience, arc tracking resistance and surface appearance.
2. Description of the Prior Art:
It is known to this art that the addition of red phosphorus into polyamide-based compositions imparts a high degree of fire resistance thereto.
U.S. Pat. No. 3,883,475 describes compositions based on fire-resistant polyamide. A metal compound, advantageously cupric oxide, is incorporated in such compositions to inhibit the release of highly toxic phosphine, which is formed following a disproportionation reaction of red phosphorus under the action of the trace amounts of water which are present in the polymer and of the high temperature required for its conversion.
In French patent No. FR-A-2,367,100 an improvement is proposed, entailing the use of cadmium oxide instead of CuO; in addition to its effectiveness (at least equal to that of CuO) for inhibiting the release of phosphine, cadmium oxide also provides articles exhibiting a particularly high arc tracking resistance (within the meaning of the NF standard C 26220), which is equal to or higher than 400 volts in the case of unfilled compositions and 375 volts in the case of compositions filled with approximately 40% by weight of glass fibers relative to the weight of the polyamide of the compositions.
In French patent No. FR-A-2,553,783 a replacement is proposed for cadmium oxide (which exhibits a certain toxicity), namely, a lanthanide-based compound. With this adjuvant, the compositions based on fire-resistant polyamide which are obtained result in articles presenting a compromise of properties, in particular with regard to fire resistance, resilience and arc tracking resistance, which is better than that associated with the use of cadmium oxide.
However, serious problem remains concerning the appearance and the development, to a variable degree, of whitish stains (whitish exudations) on the surface of articles rendered fire-resistant by means of red phosphorus, when these articles are subjected to the twin effect of a high temperature and of humidity. Such a deterioration of the articles in question is encountered, for example, in hot and humid tropical countries.
It has now unexpectedly been found that the fire-resistance of the compositions described in FR-A-2,553,783 can be further enhanced and the appearance and development of stains avoided, without appreciable disadvantage to the compromise in values of the properties in respect of fire resistance, resilience and arc tracking resistance, by incorporating therein a judiciously selected hydrotalcite stabilizer.